This invention relates to a constant power output electric power supply and more particularly to a DC power supply whose output is maintained constant in the face of changing load and line voltage conditions.
In certain applications, notably arc lamps in medical applications, it is highly desirable to deliver a constant rate of DC power. Each lamp has a power rating provided by the manufacturer. If energy delivered to the lamp exceeds its rating, the life of the lamp will be shortened. If the power delivered falls below the lamp's rating, the gas within the lamp will drop below its proper operating temperature, and this will tend to coat the inside of the envelope with a residue thereby reducing the light output, also effectively reducing the life of the lamp. As these lamps are quite expensive, it is highly desirable to maintain power levels within a fairly narrow range.
Maintaining flow of power to such a lamp is made more difficult by the fact that there is a tendency of the impedance of a gaseous discharge lamp to vary. This can occur in lamp to lamp because of manufacturing variables, it may vary because of the type of use, i.e., continuous versus intermittent use, and the impedance can change over a longer period of time due to aging.
Another problem has to do with switching from one kind of a lamp to another. Some lamps, typically xenon, may require 150 watts of power, whereas other lamps, such as mercury, may require 200 watts. In the event the operator fails to adjust the supply accordingly when the type of lamp is switched, lamp life can be shortened drastically.
Power supplies currently available for delivering a constant DC power under the conditions described above are heavy, weighing typically 40 to 50 pounds and are quite inefficient. Such devices generate substantial amounts of heat and usually require some provision for cooling, usually fans, adding to their inefficiency.
In U.S. Pat. No. 3,486,071 to Hedge, a more efficient arrangement is illustrated for delivering power to a xenon lamp operating to supply flashes of light, an electronic stroboscope. In this patent, a circuit is provided to deliver pulses of energy which are of equal power, the purpose being to compensate for changes in the voltage being supplied.
In U.S. Pat. No. 3,611,021, current to fluorescent lamps is regulated by adjusting the frequency of an inverter. In U.S. Pat. No. 4,170,747, lamp intensity is varied by altering the "on" time of a voltage source which is pulsed. Other U.S. patents showing control circuits for gas discharge lamps include U.S. Pat. No. 4,127,798, for minimizing energy surges; U.S. Pat. No. 3,754,160 for a solid state inverter to reduce power consumption at ignition; and U.S. Pat. No. 4,288,724 for an impulse generator which replaces the ballast.
None of the aforementioned patents, while providing for the control of gaseous discharge lamps, addresses itself to the problem of maintaining a constant power input into a lamp which is in continuous discharge, and providing for automatic adjustment in power delivery when a lamp with different power requirement is substituted.